positive or negative pressure in a darkroom, which is best

Discussion in 'Darkroom Equipment' started by trudee yama, Nov 10, 2006.

  1. trudee yama

    trudee yama Member

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    Building my dream darkroom/office/storage, it'll be 13'6" x 16'. Need information on several fronts: I live in Hawaii and need an air conditioner to cool and dehumidify. Should I use a ductless air conditioner and allow it to filter the air, or indoor/outdoor air conditioner and create positive pressure? Also, do I need to use a light tight exhaust fan or louver with an air conditioner or will an air conditioner be sufficient? Am also concerned about the chemicals interacting with the copper/aluminum air conditioner parts.

    Lights: Lighting the darkroom with Xenon lights, any comments would be appreciated....ie. which bulbs are best, do they create enough light?
    ANd, suggestions on lights to inspect either wet/dry prints.

    Thanks plenty,
     
  2. Mick Fagan

    Mick Fagan Subscriber

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    With air conditioning in my darkroom I have a small wall unit which brings in fresh air, slightly. From my experience you will not get enough fresh air just from the air conditioner.

    I also have a 300mm exhaust fan pulling air out via a light tight box of my own manufacture (plywood) situated above the sink.

    This has been my set-up for about 18 years. I am however seriously thinking of getting far more air movement via a separate system, maybe using something like woodworkers use for removing wood dust and shavings from their machinery. That is, an impeller type of fan hooked up to 100mm (4") pipe.

    This has come about as I am starting to notice a lack of fresh air for extended periods in the darkroom. It has never worried me before but after talking to my doctor and a health professional who specialises in clean air or lack thereof, I think it will be a wise and not so expensive decision.

    Mick.
     
  3. Roger Hicks

    Roger Hicks Member

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    Dear Trudee,

    I had my first darkroom about 40 years ago and have had at least half a dozen since (it depends on how you count them). I am a STRONG believer in good ventilation. The current one is long and thin and uses very slight positice pressure, with a more powerful impeller at one end and a less powerful expeller at the other. There are pictures of the layout in 'Our darkrooms' in The Photo School at www.rogerandfrances.com.

    Note: we also use a dehumidifier because the darkroom is an old wine cellar, built into the side of a hill.

    Cheers,

    Roger
     
  4. Bob F.

    Bob F. Member

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    One potential problem with using positive pressure is that fumes may leak in to the house around the door and under floorboards etc as well as out the intended exit. Better to have the fumes extracted close to their source and sucked straight out rather than have them floating about in the air to escape wherever they can find an exit. My extractor fans are at the end of the wet bench - a hood would be better, but that has been on my to-do list forever...

    Cheers, Bob.
     
  5. Mark_S

    Mark_S Subscriber

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    Trudee
    I live in a climate where we never need air conditioning, my darkroom is in the basement, but here is how I have it set-up as far as ventilation goes:
    I have an impeller type fan which is mounted in a box in the house, and brings air from the house down into the darkroom via a filter - this way in the winter it is heated air (something you probably don't worry about). There is also a vent that goes to the outside, with a fan which most of the time is left off. The result is that normally Fresh air is brought into the darkroom from the house, and is then vented outside. This keeps positive pressure in the darkroom which helps to keep the dust down.

    When I am doing something that is creating nasty fumes, I will turn on the exhaust fan to help vent the fumes outdoors more quickly. The fans are sized such that the fan bringing air into the darkroom is bigger than the one sucking it out.
     
  6. Monophoto

    Monophoto Member

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    Trudee -

    Having good ventilation in a darkroom is important - in the absence of fresh air, you will tire much more quickly, and the quality of the work you do will suffer.

    That said, you don't need a system that creates gale-force wind. The key is air exchange - bring fresh air in, and let stale air leave. The metric is how frequently the system will exchange the air in the darkroom.

    Assuming a standard 8' ceiling, your darkroom will have about 1730 cubic feet of volume. Suppose you use a system that has a rated flow of 300 cfm - that says that the air will be completely exchanged every 5.75 minutes - which is actually not too bad. Practically, a real installation will rarely achieve the rated flow, so a more realistic exchange time might be 10 minutes - and again, that's not too bad.

    You didn't say what kind of work you plan to do in your darkroom. For conventional processing of film and paper (developer, stop, fix and wash), the fumes are not especially harmful to humans. Toning is the process that is most likely to introduce noxious fumes into the darkroom - selenium not especially bad, but some of the sulfur-based toners (sepia and brown) smell awful and the gasses they give off can fog unprocessed film and paper. My experience is that the risk of fogging is overstated, but it probably pays to be cautious.

    You have raised the question of whether the ventilation system itself can be damaged by the darkroom fumes that it is exhausting. I think there may be two answers here - one answer is that the fumes can damage some of the components of the ventilation system (especially aluminum). The other answer is that the concentration of fumes needed to damage the ventilation system is so high that you would probably be asphyxiated before the equipment is harmed, and in reality the fumes will be so diluted by the volume of air in the darkroom that they are not going to do any damage.

    But what that says is that the key to everything is getting fresh air into the darkroom.

    My personal preference is for positive pressure because that approach helps manage dust better than a negative pressure system. However, it is often easier to install a negative pressure system.

    In Hawaii, its probably not difficult to design your system to exhaust outside the building. That's always the best approach. In my darkroom, I have a blower that pressurizes the room, and I allow air to exit via an exhaust louver over the sink. If you choose to have both an impeller and an expeller (to use Roger's terms), note the subtle point he makes that flow rate of the impeller should be greater flow rate of the expeller to achieve positive pressure.

    Final point - most "air conditioners" cool air within the room. For your darkroom, you want a system that will cool air that is brought in from outside the room.
     
  7. Neal

    Neal Subscriber

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    Dear Trudee,

    If I were building a dream darkroom, I would have an exhaust system where the hoods were just above and behind my trays and a filtered vent to allow makeup are inside. The airconditioner would be set to recirculate the air in the room.

    There is a photograph in the photoguide form Photo Techinques that illustrates the sweetest setup you could imagine. It even has continuous tray agitiation!

    The guide is available for $5 plus shipping at: http://www.phototechmag.com/shop/shopdisplayproducts.asp?id=10

    Neal Wydra
     
  8. resummerfield

    resummerfield Subscriber

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    I would strive for positive pressure, which will cut down on dust. Use the a/c unit (filtered) to supply positive pressure, and then add a powered vent over the sink. To keep the room pressurized from the a/c, size the powered vent fan to be less than the incoming a/c, or put the powered vent on a dimmer switch. My powered vent is in a plenum when extends over the back edge of my sink, and so extracts air from over the sink.

    You don’t need many white lights in the darkroom. My overall illumination is from 2 halogen floodlights aimed at white walls for indirect lighting. For print viewing I use a 60w lamp at about 4 feet, which I’ve put on a foot switch so I can turn it on with wet hands. Safelights should be on a separate switched circuit. Xenon lights may be brighter, so don’t over lamp.

    Concerning the electrical plan….. You can not have too many outlets. I have 3 branch circuits of 120v and one of 240v, 18 duplex receptacles in all, and use them all at times.
     
  9. JBrunner

    JBrunner Moderator Staff Member

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    For inspecting prints- the best lighting is what most resembles the light you expect the print to be displayed in, both in temperature and intensity. Before I commit to a run of prints, I inspect a prototype print after it is dry, in several different lighting situations found about my home, and sometimes this makes minor adjustments obvious- well worth the effort IMO.

    For ventilation, turning the air over on a regular basis is more important than positive or negative pressure, in a humid area such as you live. You will have little problem with dust, because of the humidity, so do whatever is most efficient and economical. In a dry climate things change.

    Humidity will be far more detrimental to your air conditioner, than the chemical vapors in a darkroom. Expect to replace it on about the same cycle as any other similar unit in your area. My window cooler is mostly light tight, but I did need to construct a large baffle to go over it, to completely seal off the light. You have to make it big and well ventilated so the air conditioner can dissipate the heat it generates, as is the case with mine, or if you have room inside a constructing a simple black snoot with a turn might do the trick.
     
  10. jstraw

    jstraw Member

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    In principle, the dust/positive-pressure equation sounds good but unless I always keep the door to the darkroom closed (I'll strive to) and always keep the fans running (which I absolutely do not do, they'll only run when I'm using the darkroom), then for me, the reason for positive pressure is for making sure the exhaust half of the equation is effective. Positive pressure means my exhaust fan is not starved for air.
     
  11. PeterB

    PeterB Subscriber

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    Mick, you won't get enough from a wall mounted one, but you will from a ducted system. When we had our air conidtinoing system installed I specified to the installer that at least 100 [L/s] of air flow should be vented into my darkroom from the roof mounted outlet (in one corner of the room)

    I looked into wall mounted units, but unfortunately they usually mix a certain ratio of outside air with air from the room so they can more effectively maintain the temperature. This also means that the flow rates of fresh air into the room to create a positive pressure won't be sufficient.


    regards
    Peter
     
  12. PeterB

    PeterB Subscriber

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    Hi Bob,

    I have +ve pressure in my darkroom, along with my own plenum design (cylindrical extraction tube) and an air sheet. See here.

    Without an air sheet one's ventillation system is not as effective as it could be.

    regards
    Peter